AJS & Matchless Owners Club

I recieved a Gandini piston (purchased AMOC) for my 350cc Dutch army G3LS 1956, when I opened the box their was a warning written on the lid. These pistons would warm up rapidly and result in a seizure when not allowed enough clearance. The piston inscibtion is 69.05 so its a bit bigger than expected, measuring at the skirt it measures 69.0109mm (20°C) The allready bored cylinder measures 69.0448 (3D), so much to narrow. Does anyone has experiance with these Gandini pistons, what do they advise for this combination? Thied to get in contact with Gandini, but in vain. They have a 69.00mm piston as well suitable for Matchless G3 military, did Italy use G3's after the war? If anyone have more info about these items I would be happy.

This is a very generic warning that too small a clearance will seize piston
I believe that 1953 350 had a wire wound piston with very small clearance of .001" , but your replacement is not so requires a larger clearance
Rule of thumb for clearance on modern replacements would be 1 to 1.5 thou per inch of bore , so for a 2.716" Piston ( 69.01mm) I would hone to achieve 0.00375 to 0.0040" then still run in carefully . Hepolite always recommended 0.001 - 0.0015" per inch of bore . Gandini usually require the upper limit

to give you confidence norton GPM pistons are 0.004 to 0.0045 on a 73mm bore
https://andover-norton.co.uk/files/3ba/ ... 0Twins.pdf

Thanks a lot for the info, is there an english made piston for sale? Does Omega have pistons for Matchless?

not much available for Matchless at the minute . options are:
JP piston from Australia , also available from the club in most cases
Gandino (GPM) Italian piston were the main supplier to the club but I think they are running low on stock if not run out for your bike
Omega are mainly bespoke production and hence very expensive for one off
Ebay for NOS
Russell motor still have some originals but nor sure on what oversizes

Vink27A wrote: ↑Tue Jul 18, 2023 7:09 pm The allready bored cylinder measures 69.0448 (3D), so much to narrow.

Is the piston that was in the engine a wire-wound type?

Your query SEEMS to suggest that the cylinder has already been bored prior to ordering the piston, I may be wrong, but that is the wrong way to go about it. I have never had any problems with Gandini pistons. The warning on the packaging is a standard warning ( as already mentioned ). A good engineer will hone or re-bore to the correct clearance. Why look for another make.

1608 wrote: ↑Wed Jul 19, 2023 1:58 pm Your query SEEMS to suggest that the cylinder has already been bored prior to ordering the piston, I may be wrong, but that is the wrong way to go about it. I have never had any problems with Gandini pistons. The warning on the packaging is a standard warning ( as already mentioned ). A good engineer will hone or re-bore to the correct clearance. Why look for another make.

I was just about to say the same thing.
When I had mine re-boreed I supplied the piston and the engineering firm bored it to the supplied piston..

Yes you are correct regarding the bore process, the bike was handed to me in bits and pieces, also with a rebushed and bored cylinder but without a piston, so I had to buy one afterwards. I asked 2 shops for honing the cylinder but they have to take out at least 0.08mm to get to 69,12mm, they have to do it manually, but with some risk of creating a shape deviation in the inner section.
So I wonder why the previous revison company bored it to 69.04mm, may it be correct dia for the original piston?

Vink27A wrote: ↑Wed Jul 19, 2023 5:01 pm So I wonder why the previous revision company bored it to 69.04mm, may it be correct dia for the original piston?

Not a direct help but are you aware of the Army Inspection Standards for the WD 350 which covers bore and piston sizes?
If not, it's here:

http://archives.jampot.dk/book/Workshop ... ndards.pdf

Here's just a snip of the page with cylinder dims.......

Vink27A wrote: ↑Tue Jul 18, 2023 7:09 pm The piston inscibtion is 69.05 so its a bit bigger than expected, measuring at the skirt it measures 69.0109mm (20°C) The allready bored cylinder measures 69.0448 (3D), so much to narrow.

What are you using the measure? I have a SIP305M with a Heidenhain glass scale and that can get into the 0.1µm on both external and internal measurements but you need to 0 and 90° otherwise you can't see the error in roundness.

That aside, the bore is 69.045 mm and you have a piston of 69.011 mm giving you 0.034mm (0.0014") of clearance. I did start putting all the piston data into a spread sheet but I am in Dartford at the moment and away from my PC.